In the field of aircraft and airplanes in particular, movement on the ground is very generally achieved by running on a landing gear comprising wheels, under the action of the aircraft engines or of a towing vehicle.
A distinction is usually made between the towing performed in order to leave a parking area when the airplane needs to move backward or be moved around without using its engines, running on the taxiways which is performed at low speed but using the propulsive force of the aircraft engines either to move to the start of the takeoff strip before takeoff or to return to a parking area after landing, takeoff which is running at high speed and during which the airplane propelled by its engines accelerates from a zero speed up to the speed at which the wheels leave the ground, and finally landing when the airplane comes back into contact with the ground at a speed compatible with the lift thereof and decelerates along the strip down to a speed that is low enough that it can turn off onto a taxiway.
This wheeled landing gear solution proves penalizing in that the airplane needs to have a heavy and complex landing gear that retracts when the airplane is in cruising flight at a speed higher than a few hundred km/h, and is of use only for a very short period of time during the mission, and in that use is made of the propulsion engines for running along at low speeds for which these engines, for example jet engines, are completely unoptimized. However, in the absence of more satisfactory solutions, almost all present-day airplanes are designed on this principle.
In order to improve performance, the use of ancillary devices has been contemplated.
Thus, it has been envisioned that certain wheels of the landing gear be equipped with motors which, by allowing low-speed running, avoids using the propulsion engines when moving along the taxiways. This solution though has the deficiency of making the landing gear even more complicated and even heavier than a conventional landing gear and has not yet been materially embodied in the operational use of aircraft.
Catapult systems have also been developed to assist airplanes with takeoff. The catapult supplies the airplane with additional energy during acceleration on takeoff by towing the airplane with a suitable force.
Such a system, which is built into the takeoff strip makes it possible to shorten the distance run during takeoff, and this has justified its widespread use on aircraft carriers. However, a catapult is a device that proves restrictive from an operational standpoint and requires a great deal of power in order to develop the desired amount of energy over a very short space of time. A catapult has only one functional direction, something which does not impede its use on an aircraft carrier which always heads into the wind for aircraft takeoff, but in practice would mean installing a catapult in each direction of the strip in the case of a “fixed” strip. Given the various constraints of such a system, airplanes enjoying takeoff assistance using a catapult are currently limited to a few tens of metric tons at most.